(ôl'tər-nā'tĭng) An electric current that repeatedly changes its direction or strength, usually at a certain frequency or range of frequencies. The term is also used to describe alternating voltages. Power stations generate alternating current because it is easy to raise and lower the voltage of such current using transformers; thus the voltage can be raised very high for transmission (high voltages lose less power as heat than do low voltages), and lowered to safe levels for domestic and industrial use. In North America, the frequency of alternation of the direction of flow is 60 Hz, or 60 cycles per second. In other parts of the world it is 50 Hz. Compare direct current. See Notes at current, Tesla.

An electric current in which the flow reverses periodically. (Compare direct current (DC).)

Note: In the United States, most household current is AC, going through sixty reversal cycles each second. Electric motors in household appliances are designed to work with current at this rate of reversal.